1. Field of the Invention
The present invention relates to MOS integrated circuits having high integration density.
2. Description of the Prior Art
Current trends are directed towards the fabrication of integrated circuits in which it is continually sought to achieve further reductions in size while at the same time incorporating the greatest possible number of elements. This entails the need to produce individual element structures which are as small as possible and to make use of integration technologies with self-alignment of the patterns which are formed successively on the circuit wafers.
In particular, one technique which has now found wide acceptance lies in the use of MOS transistor gates formed of doped polycrystalline silicon and etched prior to implantation of the source and drain regions of the transistors. The gate thus serves as a mask at the time of implantation. The source and the drain are therefore self-aligned with respect to the edges of the gate.
Polycrystalline silicon layers are also employed in the fabrication of source and drain electrodes in contact with the monocrystalline silicon of the substrate and for the formation of connections between transistors. Apart from the ability of doped polycrystalline silicon (of the same conductivity type as the source and the drain) to serve as a mask at the time of implantation of the source and drain regions, this material offers the advantage of establishing a good ohmic contact with the source and drain regions.
Unfortunately, even if polycrystalline silicon is heavily doped, its conductivity is relatively limited and consequently needs to be increased (since it serves to form conductive connections between elements). To this end, a layer of metallic silicide (platinum silicide, titanium silicide, tantalum silicide, for example) is formed on the top surface of the polycrystalline silicon.
This layer is formed by depositing a metallic layer above the polycrystalline silicon (by cathode sputtering from a target of the metal considered) and by annealing so as to form an alloy (silicide) with the superposed polycrystalline silicon. A silicide can also be deposited directly by cathode sputtering from a target of sintered metallic silicide. This depends on the metal considered.